I know that there is Core Data to store information onto your phone, but what if you are making a social media application such as Instagram. Where did they manage to store all the information for every user? My main question is when there is a large amount of data to be stored for an application where do application designers store this information? Thanks

PHP and MySQL are standard and low-end languages and databases to use and quite easy to learn and become familiar with. More advanced databases tend to use caching layers and other types of layers over the database which prevent data loss, increase read and write speeds, cache the request data, and more. Furthermore, other companies may chose to use NoSQL databases.